Tag Archives: time between overhaul

Fashionably Late Doesn’t Apply To Your Engine TBO. Here’s Why

This article originally appeared on the P&WC Airtime Blog.

There are two simple reasons why always respecting an engine’s TBO is of fundamental importance to any operator: performance and economics. Discover why this is one deadline you don’t want to miss.

Continue reading Fashionably Late Doesn’t Apply To Your Engine TBO. Here’s Why

General Information about Aircraft Engine Overhaul

There are variables which can alter the TBO for an aircraft engine.


TBO (Time Between Overhauls) is determined by the manufacturer of the engine.  It is meant to suggest the service life between overhauls when flown at a minimum interval (40 hours per month for one manufacturer).  Aircraft power plants love to fly, the purpose for which they are built. If they are used less frequently than this suggested interval, they will become prone to moisture build up and corrosives in the oil, shortening the service life of the motor when not brought to operating temperature on a regular basis.  Also, some engines will be exposed to extreme conditions that should be taken into account.  Life for an engine operating primarily in dry, dusty conditions, near salt water, in varied climates or any combination of these is subject to extra wear.  There is a reason planes don’t come with roadside assistance. It’s the same reason to adhere to the TBO.

The reason for aircraft engine overhaul isn’t necessarily to prevent cataclysmic failure of the power plant.


An engine is built to tolerances.  When it is “fresh,” such as when new or after an overhaul, the tolerances are very close.  Normal wear on the metal parts causes these tolerances to widen.  There is a point in the design of the motor where the tolerances increase and performance is affected.  When this happens oil consumption can increase, power can decrease and the risk of failure of an engine component increases.  Overhaul is as much about inspection of the engine parts as it is about repair or replacement and this cannot be done during any other time of aircraft maintenance.  Inspection of the components and their interactions can play a major role in the diagnosis of a problem before it causes failure.  Inspection is the most precise and important part of an overhaul and cannot be performed in a careless manner.

What is entailed in an aircraft engine overhaul.

The term “engine overhaul” means the process of maintaining and restoring equipment, machines or systems to serviceable conditions.  This involves the disassembly, inspection to detect damaged, defective or worn parts, the repair or replacement of these parts and the reassembly, testing and trial-run prior to the return to its full operating level.  There are two types of overhaul: major or top overhaul.

What is performed in a major overhaul?

In a major overhaul, the engine is completely disassembled.  Every part is inspected, repaired as necessary, reassembled, tested and approved for return to service within the fits and limits specified by the manufacturer’s overhaul data.  This could be a return to the fits and limits of new, or to serviceable limits.

What is performed in a top overhaul.

The top overhaul consists of repairs to parts outside of the crankcase.  This can be accomplished without the complete disassembly of the engine.  It can include the removal, inspection and repair of cylinders, inspection and repair of cylinder walls, pistons, valve-operation mechanisms, valve guides, valve seats and the replacement of pistons and piston rings.  Reassembly and testing follow this to ensure that the return to service life is trouble-free.

The TBO is more than just a suggestion.

The life of aircraft engines isn’t easy.  There are many factors that come into play when determining the serviceable life.  The metals inside go through a great deal of heating and cooling cycles due to the internal combustion process as well as the cool-down period after flight.  Metals begin to degrade over time due to these cycles.  Metal-to-metal wear due to improper lubrication and during start up (running the short time before lubrication can make it to the parts) take a toll on the engine.  The very design of an engine is to allow for expansion to tolerances after operating temperature range has been achieved. This means that until this range is reached, the tolerances are operating at less than optimal, which allows extra wear to occur should anything but the minimum of power be applied.  Add to this operational conditions, such as interval of use and environmental conditions, and the need for regularly scheduled inspection of all working parts becomes quite clear.

With so much riding on the line, the very best of care should be taken when selecting the facility to perform service on aircraft.

Covington Aircraft is a Pratt & Whitney Canada certified distributor and designated overhaul facility, offering service on all their turbine and radial aircraft engines.  We also perform annuals, 100 hours and most service-related work on any airframe.  We have flight safety personnel ready to service your needs.  If for any reason we are unable to provide a service for you, we can provide you with contact information of someone who can.  In our 47,000-square-foot facility, we offer state-of-the-art engine testing, new engine installations, complete overhaul service using the latest equipment, and we maintain a complete line of engine parts and accessories so we can get you back in the air in the shortest time possible.  We are ready to perform all stages of engine repair and overhaul work in-house on both radial and turbine aircraft engines.  When you add to this the service, dependability, commitment to quality and competitive pricing that is part of our policy, you will understand why we have been in the business for nearly 40 years.

To read more about planes, engines and how much we care about our clients, please click here.  Learn more about us and our commitment to quality and dependability by clicking here.


TBO: Time Between Overhaul. Even the PT6A Needs Maintenance

Covington Aircraft has now been in business since 1979.  Over the last 30 plus years, we have seen some Agricultural Aircraft owners who were riskier than even the most daring stunt pilots. How so? Well, they try, much like those who push the oil change on their vehicle, to put off as long as possible the overhaul of their engine. We focus on what to look for and Time Between Overhaul in this post.


The Ag industry often overlooks FAA Part 91/137 (Agricultural Operations) regarding recommendations for TBO, or Time Between Overhaul.  TBO is overhauling an engine at the manufacturers recommended times.  Often, Ag industry pilots are under the mistaken impression that PT6A engines don’t need these recommended overhauls, when this couldn’t be further from the truth.

SB 14503 (-67AG)

Pratt Service Bulletins governing TBO for this engine indicate that the first stage power turbine blades should be replaced at 12,000 hours, while second stage power turbine blades should be replaced at 5,000 hours.  Compressor Turbine Blades with certain part numbers must be replaced at 6,000 hours.  Main-line rotor bearings #1 and 4 as well as the first stage planet gear set’s sun gear should be replaced at 12,000 hours time.  Considering these low hours, it stands to reason that the PT6A will not fly forever if the recommendations are not followed.

PT6A-34AG SB 1303

pw-pt6aThe 5000 Hour Check, which involves removing the disc from the hot section, removing blades from the disc, cleaning and conducting non-destructive testing, is recommended for the first time at 5,000 hours and at 3,000 hour intervals after that.  This inspection also requires comparison of the actual length of the blade but they cannot be cracked.  Cracked turbine blades or those stretched beyond limits could be devastating to an aircraft.

These two simple recommendations indicate that PT6A engines do not last forever.  Failure to follow these recommendations could result in engine failure.  In addition, Pratt & Whitney places significant importance on maintenance recommendations when honoring warranty repairs.  Therefore, it is important that owners educate themselves on service bulletins and overhaul recommendations to avoid catastrophe.

If it has been too long of a TBO for you or you have general maintenance, sales, or overhaul questions, make sure you visit the Covington Aircraft website!  Happy Flying!